1. Field of the Invention
The present invention is directed to a method for image generation by means of magnetic resonance, of the type wherein a number of independent reception antennas having sensitivity profiles differing from one another are employed for generating an overall image.
2. Description of the Prior Art
PCT Application WO 99154746 discloses a method of this type wherein radio-frequency excitation pulses and gradient pulses are emitted in an imaging region, the imaging region being divided into partial imaging regions, for generating location-coded magnetic resonance signals, with the gradient pulses including phase-coding gradients for the location coding in a phase-coding direction, with the location coding in phase-coding direction being incomplete. The magnetic resonance signals are simultaneously received with the reception antennas, and a k-space dataset is formed from the reception signals of each reception antenna.
An intermediate image is reconstructed from each k-space dataset. The intermediate images include fold-over artifacts due to the incomplete location coding in the phase-coding direction.
A weighted combination of the intermediate images ensues with weighting matrices allocated to the antennas to form a fold-over artifact-free, overall image.
The time required for the generation of a magnetic resonance image using measurement sequences that are standard, and with a given size and resolution of the image is determined by the intensity of the gradient magnetic field employed for the topical resolution. Although the gradient coils with which the gradient magnetic field is generated are becoming increasingly powerful and the measurements are becoming increasingly faster as a result thereof, a physiologically prescribed limit (stimulation limit) that cannot be exceeded exists because of the magnetic fields that are rapidly switched and due to the electrical voltages that are induced in the tissue of the patient as a result.
In recent years, methods have been developed that are referred to as coil sensitivity encoding methods or partial parallel acquisition (PPA). These methods use sensitivity profiles of the individual antennas of an antenna array in order to reduce the phase-coding steps required for the topical resolution, and thus shorten the measurement time.
The article by Hutchison and Raff, xe2x80x9cFast MRI Data Acquisition Using Multiple Detectorsxe2x80x9d, Magnetic Resonance in Medicine, Vol. 6, pp. 87-91 (1988) discloses a method wherein only one phase-coding step is required for the production of an image. An antenna array is used that has a number of independent individual antennas and radio-frequency channels that corresponds exactly to the number of phase-coding steps in conventional sequential phase coding with phase-coding gradient fields. Due to the high required number of reception channels, this method is difficult to practically implement.
The article by James R. Kelton, Richard L. Magin, Steven M. Wright, xe2x80x9cAn Algorithm for Rapid Image Acquisition Using Multiple Receiver Coilsxe2x80x9d, Proceedings of the SMRM 8th Annual Meeting, Amsterdam, 1989, p. 1172, discloses a measurement method wherein the idea of Hutchison and Raff was expanded. The number of individual antennas in the antenna array amounts to a power of two therein. The measuring time is shortened dependent to this number of antennas. The number of independent radio-frequency reception channels can be selected significantly lower than the number of phase-coding steps otherwise required for the image determination.
As noted above, a parallel acquisition method of the type initially disclosed in PCT Application WO 99/54746. For determining the antenna sensitivity profiles required for the reconstruction of the final image, a reference measurement with the same or even with a lower resolution than in the actual image production is implemented before the actual exposure. To that end, the magnetic resonance signals are measured with the individual antennas in the antenna array as well as with the whole-body antenna permanently installed in the magnetic resonance apparatus. The sensitivity profile of the whole-body antenna is constant enough in order to be used as a reference. The complex images (in the mathematical sense) of the individual antennas obtained after the Fourier transformation and the reference image of the whole-body antenna are placed into relationship with one another, and the complex (in the mathematical sense) sensitivity profiles of the individual antennas are obtained. After determining the weighting matrices from the antenna sensitivity profiles, these weighting matrices are then employed for reconstruction in the following, actual measurement.
For the parallel acquisition technique, it is important to identify exactly the antenna sensitivity profiles employed for the reconstruction from in vivo measurements. The intensity of the magnetic resonance signals from voxels corresponding to the picture elements in the examination region is decisive. The calculation of the antenna sensitivity profiles is no longer trivial for picture elements that represent only weakly imaging tissue.
Heretofore, the signal intensity of the magnetic resonance exposures from a reference scan or a pre-scan, used for determining the weighting matrices was compared to a threshold in order to determine whether imaging tissue was present for the presentation of the picture element in question. When the signal intensity is higher than the threshold, i.e. when imaging tissue is present, the antenna sensitivity profiles and the weighting matrices are calculated from the measured signals in order then to be employed for the reconstruction. Conversely, when no tissue or only weakly imaging tissue is present at the picture element in question, the picture element must be estimated by interpolation or extrapolation. The aforementioned PCT Application WO 99/54756 discloses a possibility for interpolating or extrapolating the missing antenna sensitivity profiles from the neighboring picture elements. This method, however, also involves some disadvantages. When the threshold lies very high, many picture elements are interpreted as noise and these picture elements must be interpolated or extrapolated. The interpolation or extrapolation is very difficult to implement in this case. This can lead to fold-over artifacts in the final image that are still visible after the reconstruction. When the threshold lies very low, the calculated antenna sensitivity profiles are very highly influenced by noise. The reconstruction no longer supplies weighting matrices that are optimized in terms of signal-to-noise. The signal intensities outside the patient contour must be extrapolated because of the lack of imaging substance. This extrapolation can be unstable. Further, the calculation is correspondingly involved in terms of calculating time.
An object of the present invention is to provide a method for fast image generation, wherein the weighting matrices for the fold-over artifact-free overall image can be dependably determined.
The object is achieved in a method as initially described wherein the weighting matrix is determined from a fold-over artifact-free intermediate weighting images acquired with the reception antennas, such that noise and the sum of fold-over artifacts due to incomplete location coding in the phase-coding direction is minimized in the reconstructed overall image, with the overall image being composed of the intermediate weighting images weighted with the elements to be defined in the weighting matrix.
It is advantageous that the antenna sensitivity profiles now no longer need to be smoothed or extrapolated, since errors in the extrapolation can lead to artifacts in the final image. Further, the signal-to-noise loss that is otherwise present due to parallel acquisition can be reduced. In known parallel acquisition methods, the ratio of signal to noise is optimized dependent on the condition that the fold-over artifacts are completely suppressed. In the present case, in contrast, the ratio of signal to noise is optimized together with the fold-over artifacts. The signal-to-noise ratio thus is improved for image regions having only slight fold-over artifacts.
In an embodiment the fold-over artifact-free intermediate weighting images for determining the weighting matrices and the magnetic resonance signals for the overall image are acquired with the same image sequence, with the location coding in the phase-coding direction being complete in the low-frequency region and incomplete in the adjacent higher-frequency region. A separate pre-scan for determining the weighting matrices thus can be foregone; the weighting matrices are determined from the middle region of completely occupied k-space. When the middle region of k-space and the adjacent higher-frequency regions are occupied with the same sequence type, differences in the contrast of the images for determining the weighting matrices and for producing the actual magnetic resonance image do not occur.